1. Field of the Invention
The present invention relates to the field of filter analysis and design and, more specifically, to systems and methods relating to tuning filters.
2. Description of Related Art
The past few decades have seen considerable advancement in electronics and wireless communications. The continued development and advancement of more highly dense integrated circuits at low cost has enabled a plethora of mobile devices, and particularly wireless mobile devices, to become prevalent around the world to the point of being ubiquitous. Mobile devices having wireless capability and found throughout the world today include, for example, mobile telephones, personal digital assistants (PDAs), laptop computers, global position sensor (GPS) devices. These devices typically operate in the radio frequency (RF) and microwave wireless signal frequency ranges.
The electronics for communicating at RF and microwave frequency requires transmitters and receivers with electric signal filters to assist in producing and/or discriminating between wanted signals and unwanted signals. However, it is difficult to build an electric signal filter for wireless communication that has ability to discriminate between wanted and unwanted signals as well as desired. Therefore, the electric signal filters are tuned after being made or manufactured, so that they are better at producing and/or discriminating between wanted and unwanted signal frequencies.
Electric filters for wireless communication include, for example, cavity type filters and planar type filters. Electronic filters such as the planar filter may include a series of resonators coupled together. High performance planar filters, for example high temperature superconductor filters (HTS), have been developed to provide extremely accurate filtering to improve the quality of wireless communications, particularly in areas having a high density of wireless devices or where the RF or microwave signals may not propogate well. See, for example, U.S. patent application Ser. No. 10/944,339 “Stripline Filter Utilizing One or More Inter-resonator Coupling Members” which is hereby incorporated herein by reference for all purposes.
Planar filters are usually patterned on high dielectric constant substrates and designed to be very compact in size. Using the precise lithography techniques developed for semiconductor processing, couplings that are well repeatable within in acceptable range can be produced. Unlike cavity filters, planar filters do not generally require tuning of the couplings because the filter response is less sensitive to coupling variations than resonant frequency variations. However, substrate thickness variations and/or process variations such as etching conditions are likely to cause unacceptable resonant frequency variations of planar filters, and thus require tuning of planar filters.
Several tuning techniques have been used for planar filters, for example high performance superconductor filters, have been developed. Maintaining high-performance in the filter design stage or in manufacturing requires a stable tuning process. There are two main approaches to planar filter tuning. The first approach, mechanical tuning, is widely used in the industry. Filters may be tuned mechanically by moving elements such as dielectric rods or conductive tips within the electromagnetic field near resonators. For example, tuning screws may be used to move the dielectric rods or conductive tips up and down over the resonators. For superconductor filters, sapphire rods or superconductor-coated tips may be used on the tuning screws. Sapphire rods may placed at high electromagnetic field area over resonators and tune resonant frequency by changing shunt capacitance to ground. Superconductive tips can be used for magnetic and/or electric field tuning, but usually they are applied to the electromagnetic field because it can tune more effectively. The tip changes the electromagnetic field surrounding the resonator(s) and varies inductance of resonator(s). One exemplary method of providing mechanical tuning is described in U.S. Pat. No. 5,968,876 by Sochor, which is hereby incorporated by reference herein for all purposes.
One advantage of the mechanical tuning approaches is reversibility. Filters are tuned through a trial and error process by moving the tuning elements or screws up and down. Later on, tuning still can be adjusted if it is necessary. One disadvantage of mechanical tuning is that the tuning elements or screws can potentially impact the resonant frequencies of other resonators or inter-resonator couplings when they are applied, especially when they are placed close to the circuit. In reality, that happens often. The variation in coupling ultimately limits the filter's tuning range. This effect can be minimized by taking it into account during filter design. Designers may arrange resonators tuning locations away from each other and away from the couplings to avoid that impact. This concern and approach limits freedom of design of planar filters. There are other issues that may be caused by having mechanical part. For example, metallic or dielectric flakes may drop from mechanical elements or screws during and after tuning. These flakes may affect the filter Q-factor and also change tuning as they are free to move around on the circuit. The tuning elements also need to be fixed or locked in location after the tuning is finished to keep the filter's performance constant.
The second approach is done by processing and does not need mechanical parts. A couple of methods, such as laser trimming a portion of the filter trace or depositing a thin dielectric layer over the filter trace have been reported. One exemplary laser trimming technique is shown in the article by Parker, Ellis and Humphreys, Tuning Superconducting Microwave Filters By Laser Trimming by Goodyear, IEEE MTT-S Digest, 2002, which is hereby incorporated herein by reference for all purposes. One exemplary dielectric deposition technique is described in the article by Tsuzuki, Suzuki, and Sakakibara, Superconducting Filter for IMT-2000 Band, IEEE Transactions on Microwave Theory and Techniques, Vol. 48, No. 12, December 2000, which is hereby incorporated herein by reference for all purposes. These approaches will result in permanent tuning changes, and should not change once they are set. Thus, there is no chance to retune or readjust the filter. Hence, tuning must be done very carefully so that the filter is not permanently ruined.
In general, the second approach is preferable to the first approach, even though the first approach is predominantly used. However, there are two major issues that must be resolved in order to realize the second approach. First, a reproducible tuning process must be developed. Second, a robust method that provides a tuning recipe is needed. Both must be very accurate since the tuning is generally not reversible. It would be beneficial if a filter design may be provided that is insensitive to trimming accuracy so as to often tune filters accurately. The present invention provides a number of approaches to filter tuning and design which meet these requirements.